1. Field of the Invention
This invention is directed to a system and to a method. More specifically, this invention relates to improvements in design and operation of gas chromatography instrumentation and improvements in gas chromatographic analysis techniques. This invention further provides an improvement in system and method for heating and temperature control of gas chromatography columns utilizing resistance heating and resistance measurements techniques. More specifically, the system and method of this invention permit direct resistive heating involving the use of fused silica open tubular capillary columns having a conductive exterior coating and means for electrically coupling a power source to such coating, whereby it is now possible to effect direct resistive heating of the capillary column while controlling the temperature thereof. Such control is achieved by measurement of the resistance of the conductive coating, correlating such resistance with the temperature of the column and increasing or decreasing the electrical power supplied to such coating, as appropriate, to effect the desired adjustment in column temperature. Because of the low thermal mass of the column and the conductive coating, temperature changes can be effected readily thereby improving the speed of such separation and reducing the interval between resolution of successive samples. In the preferred embodiments of this invention, the system is incorporated in what is referred to as "short column" gas chromatography instrumentation and thus suitable for "non-selective" detection systems or selective detection methods. Because of the relative compact nature and low power demands of the system of this invention, such system can be operated as a portable unit.
2. Description of the Prior Art
The field of gas chromatography, as traditionally practiced, and as it has evolved more recently, involves five (5) basic, functionally interrelated components: (1) a carrier gas flow control system; (2) a low dead volume sample injection system (which allows for delivery of reproducible sample size); (3) a column oven that can be used for isothermal and temperature programmed analysis; (4) a low dead volume and high-sensitivity detector for high-speed gas chromatography and trace analysis; and (5) a short-time constant recorder or a chromatographic data recorder system for accurate recording of the peaks eluded from open tubular columns.
The design and operation of such equipment has as its ultimate objective the production of columns of extremely high resolving power for the separation of complex and difficult samples.
The control over column temperature is critical to the resolving efficiency of the column, whether the system is designed to operate isothermally or by temperature programmed analysis. Such column temperature control can be achieved by heating of the column in a convection oven or by resistance heating techniques. As has been appreciated, forced air heating of a column is relatively inefficient and temperature change of the column effected at a relatively slow rate. The application of direct resistance heating techniques to metal columns affords more rapid heating than forced air techniques, however, due to the relatively substantial thermal mass of such systems (both the metal columns and the ovens within which they are heated) such systems require a relatively long cool down (equilibration) periods, (See for example U.S. Pat. No. 4,096,908; column 2, lines 37-64).
Following separation or resolution of the mobile phase within the column, it is thereupon subjected to analysis by flame ionization techniques or by mass spectrometry. Where such analysis requires the transfer of the mobile phase from the column to the separate instrument (as in the case of mass spectrometry), heated transfer lines must be provided to maintain the mobile phase at a constant temperature. U.S. Pat. Nos. 4,650,964; 4,728,776; and 4,735,259 are representative of systems for transfer of a mobile phase from a column to an analytical instrument via a heated capillary tube. In each such system, the temperature of the mobile phase within the capillary tube is maintained at the appropriate temperature during the transfer process by means of what is referred to as a "heater tube".
Because of the obvious deficiencies in the above temperature maintenance and control systems, the search for alternative heating techniques continues. The direct resistance heating of metal-coated, thin-walled, flexible fused silica columns has been proposed because of the obvious advantages such a system would provide. Unfortunately, neither the manner of implementation of such a system, nor the feasibility of such system has been reported or established. Lee, Milton L. et al., Open Tubular Column Chromatography (A Wiley-Interscience Publication, John Wiley & Sons, 1984) Chapter 4, p. 110. While the above alternative to a more traditional technique of control of column temperature appears to offer promise, other potential problems are evident from this approach (i.e., temperature control). More specifically, even if thermocouples or resistance thermometers were available and small enough to accurately measure the temperature of these columns, the placement of the sensor against the column would create a local cold spot and lead to inaccurate temperature measurement. Thus, while the fused-silica column option appears attractive, there still remains numerous problems to overcome before such technology can be implemented successfully. Therefore, there continues to exist a need for improvement in the heating and temperature control of gas chromatography columns to permit rapid and efficient programmed temperature control and yet achieve such objectives without a prolonged equilibration period between successive analyses.